1. Field of the Invention
The present invention relates to a bar-type antenna having directionality.
2. Description of the Background Art
A conventional keyless entry system is configured to allow a user to lock or unlock a door of a vehicle or the like through user's manual operation of a button of a remote unit. In this system, a low-frequency (LF) band signal is used, and a bar-type antenna is employed in view of its capability for downsizing irrespective of a level of wavelength.
As is commonly known, a typical bar-type antenna has a FIG. 8 (eight)-shaped directional characteristic. FIG. 25(a) is a perspective view showing one example of a conventional bar-type antenna. As shown in FIG. 25(a), the bar-type antenna 40 comprises a bar-shaped core 41, and a coil 42 wound around a central portion of the core 41.
In the bar-type antenna illustrated in FIG. 25(a), the core 41 has a bar shape having the following size: length L×width W×thickness T=20 mm×10 mm×10 mm.
In FIG. 25(a), the origin O is a center point dividing in half each of the length L, the width W and the thickness T of the core 42. A lengthwise direction of the core 41, a widthwise direction of the core 41 and a thicknesswise direction of the core 41 will hereinafter be referred to respectively as “X axis”, “Y axis” and “Z axis”.
Further, a point located forward of the bar-type antenna at coordinate (X, Y, Z)=(1m, 0, 0), and a point located rearward of the bar-type antenna at coordinate (X, Y, Z)=(−1m, 0, 0) will hereinafter referred to respectively as “point A” and “point B”.
The core 41 has the following properties: relative magnetic permeability μr=80, and electrical conductivity σ=0 s/m. A wire having a diameter φ of 0.3 mm is wound around the central portion of the core 41 by 20 turns, to form the coil 42.
FIG. 26 is a graph showing a magnetic field intensity distribution in an X-Y plane in a state when an AC current source I (see FIG. 25(b)) is connected to the coil 42 of the bar-type antenna 40 illustrated in FIG. 25(a). The AC current source I is set as follows: frequency f=125 kHz, and current value i=2 App.
In this case, respective magnetic field intensities at the A and B points are as follows: the point A: A=1.53×10−3 A/m, and the point B: B=1.51×10−3 A/m. A directional sensitivity (20×log(A/B)) is 0.10 dB, and thereby there is substantially no directionality.
As above, the magnetic field intensity distribution is symmetrical with respect to each of the axes of the core 41, and thereby the conventional bar-type antenna has no directional sensitivity in an axial direction of the core 41. In this connection, the following Patent Document 1 discloses a technique of combining a plurality of bar-shaped antenna elements to provide a non-directional antenna.
As a recent keyless entry system, there has been known a smart entry system configured to allow a driver to automatically unlock a door of a vehicle simply by approaching the vehicle while carrying a remote unit, and to automatically lock the door simply by getting out of the vehicle and moving away from the vehicle. Recent years, the smart entry system has also been employed in a front door of a house.
In the smart entry system, if a conventional antenna is arranged to have a receiving sensitivity in an outward direction relative to a door, the antenna will also have a receiving sensitivity in an inward direction relative to the door. Thus, in cases where the conventional bar-type antenna is employed in the smart entry system for a front door of a house, there is a problem that the front door is unlocked even when a person who carries a remote unit within the house approaches the front door to check a visitor. Therefore, in order to avoid such an unintended unlock, it is necessary to provide a difference between the respective receiving sensitivities in the outward and inward directions relative to the door. As one technique of providing such a difference in receiving sensitivity when the conventional bar-type antenna is employed in the above smart entry system, a shielding member 6 is installed in a direction from which a bar-type antenna 5 should not receive electromagnetic waves, as shown in FIG. 27.    [Patent Document 1] JP 2002-217635A    [Patent Document 2] JP 3495401B    [Patent Document 3] JP 2007-065881A
In a specific system, such as a keyless entry system, an antenna is required to have asymmetrical directionality in a forward-rearward direction thereof in some cases.
In cases where a communication distance is sufficiently greater than a wavelength, communication is performed in a radiation electromagnetic field domain, so that directionality of an antenna can be freely controlled using a conventional technique.
Even in short-distance communication, as long as the communication is performed using a high-frequency band, i.e., in a short-wavelength region, the communication is performed in the radiation electromagnetic field domain. However, in a short-distance wireless communication system using a low-frequency band, such as a keyless entry system or a smart entry system, a wavelength is significantly long as compared with a communication distance. Thus, communication is performed in an induced electromagnetic field domain.
In the induced electromagnetic field domain, it is difficult to freely control directionality of an antenna. Moreover, although a bar-type antenna may be partially surrounded by a shielding member when it is necessary to allow the bar-type antenna to have a difference between respective receiving sensitivities in forward and rearward directions in the induced electromagnetic field domain, such a technique involves a problem in terms of cost and external appearance.